In the manufacture of integrated circuits, planarization of semiconductor wafers is becoming increasingly important as the number of layers used to form integrated circuits increases. For instance, metallization layers that provide interconnections between various devices may result in nonuniform surfaces. The surface nonuniformities may interfere with the optical resolution of subsequent photolithographic steps, leading to difficulty with printing high resolution patterns. The surface nonuniformities may also interfere with step coverage of subsequently deposited metal layers and possibly cause open or short circuits.
Various techniques have been developed to planarize the top surface of a semiconductor wafer.
One of these techniques involves polishing the wafer using polishing slurry that includes abrasive particles mixed in a suspension agent. Chemical mechanical polishing or planarization (CMP) is a technique of polishing materials including semiconductor substrates and films overlying such substrates, which provides a high degree of uniformity and planarity. The process is used to remove high elevation features on films created during the fabrication of a microelectronic circuitry on the substrate, or to remove a layer of film to reveal the circuitry buried underneath the film. In some cases, the process can planarize semiconductor slices prior to the fabrication of microelectronic circuitry thereon.
A CMP slurry serves multiple roles; namely, it is the medium in which the abrasive particles is dispersed, and secondly it furnishes the chemical agents which promote the chemical process. In order for optimum results in CMP processing, there must be a synergistic relationship between the chemical and mechanical processes.
With this approach, the wafer is mounted on a wafer holder, a polishing pad coated with the CMP slurry is mounted on a platen, the pad and the wafer are rotated such that the wafer provides a planetary motion with respect to the pad, and the polishing pad is pressed against an exposed surface of the wafer with a hydrodynamic layer of the slurry there between. The polishing erodes the wafer surface, and the process continues until the wafer is largely flattened.
Chemical-mechanical polishing has become a popular wafer planarization technique. For instance, chemical-mechanical polishing is becoming a preferred method of planarizing tungsten interconnects, vias and contacts, and with proper process parameters has shown significantly improved process windows and defect levels over standard tungsten dry etching.
The slurry is prepared to the desired composition, the desired concentration, the desired purity, etc. Typically, the slurry is mixed in bulk by adding the abrasive particles and additives, oxidizers, etchants and/or de-ionized water to the suspension agent by a slurry manufacture. The slurry may be fabricated at remote locations from the end use facility.
Containers are employed as a source of process the slurry. In such containers the slurry is transported to the point of use by truck, rail or air transport. At the semiconductor facility the slurry containers are deposited or the slurry is transferred from the containers to a slurry distribution system or deposited in a separate storage tank.
However, the method of transporting or storing the slurry for polishing in a container or other device has, among others, the following three problems (a) though (c):
(a) The slurry may dry or inorganic particles in the slurry may flocculate or precipitate during the transport or storage. As a result the concentration and purity of the slurry or the particle size of the inorganic particles in the slurry may differ from those immediately after the preparation, and the desired polishing characteristics may not be obtained. If the slurry has changed its desired polishing characteristic and is used in the planarization process, this can cause to insufficient planarization results. Nonuniform surfaces on the wafer may lead to faults in the circuitry in one of the following process steps. As a result the scrap rate will increase. Very high costs will arise to semiconductor manufacturers by using such altered slurry.
(b) The desired polishing characteristics depend on the type, etc. of the semiconductor devices, etc. to be polished. For this reason, it is necessary to determine the composition, concentration, purity, pH, etc. of the slurry according to the object to be polished. However, it takes the slurry manufacturers very high cost to manufacture the slurry to the composition, concentration, purity, pH, etc. specified by the manufacturers who manufacture and polish semiconductor devices in their orders and supply the same to the manufacturers who manufacture and polish semiconductor devices on the other hand, a method of supplying the slurry prepared to the specified polishing characteristics (compositions, concentrations, purities, pHs, etc.) to the manufacturers who manufacture and polish semiconductor devices for their stock has a problem that the storage period may be longer and the above-described problem (a) may occur.
(c) The slurry which is low in stability is subject to quality deterioration during the storage or transport. Therefore, even if the polishing characteristics of the slurry has high polishing performance immediately after the manufacture, the quality of the slurry may deteriorate during the storage or transport, the polishing performance of the slurry may also deteriorate, and as a result, the slurry may no longer applicable to any industrial uses.